Approach Considerations

The diagnosis is mostly based on the clinical presentation. However, leukocytosis with left shift presentation and possibly eosinophilia a few weeks after convalescence on a standard blood test and urine tests are part of a complete medical workup. The following studies are indicated in scarlet fever:

Throat or nasal culture or rapid streptococcal test

Anti-deoxyribonuclease B, antistreptolysin-O titers (antibodies to streptococcal extracellular products), antihyaluronidase, and antifibrinolysin can be valuable in confirmation of the diagnosis

In most cases, no imaging studies are indicated.

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Blood and Urine Studies

The complete blood cell (CBC) count commonly reveals a leukocytosis. The white blood cell (WBC) count in scarlet fever may increase to 12,000-16,000/μL, with a differential of up to 95% polymorphonuclear leukocytes. During the second week, eosinophilia, as high as 20%, can develop.

Urinalysis and liver function tests may reveal changes associated with complications of scarlet fever. Said tests are part of a complete medical workup. Hemolytic anemia can occur, and mild albuminuria and hematuria may be present early in the disease.

Patients whose bacterial source may suggest another process (eg, a patient with a suppurative leg wound who may have osteomyelitis) should be evaluated accordingly.

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Throat Culture

Throat culture remains the criterion standard for confirmation of group A streptococcal upper respiratory infection. American Heart Association guidelines for prevention and treatment of rheumatic fever state that group A streptococci virtually always are found on throat culture during acute infection.
[19]

Throat cultures are approximately 90% sensitive for the presence of group A beta-hemolytic streptococci (GABHS) in the pharynx. However, because a 10-15% carriage rate exists among healthy individuals, the presence of GABHS is not proof of disease.

To maximize sensitivity, proper obtaining of specimens is crucial. Vigorously swab the posterior pharynx, tonsils, and any exudate with a cotton or Dacron swab under strong illumination, avoiding the lips, tongue, and buccal mucosa.

Direct antigen detection kits (ie, rapid antigen tests [RATs], strep screens) have been proposed to allow immediate diagnosis and prompt administration of antibiotics. Kits are latex agglutination or a costlier enzyme-linked immunosorbent assay (ELISA). Several studies of RAT kits report results of 95% specificity but only 70-90% sensitivity. Operator technique can also significantly influence the results of the test.
[20]

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Antideoxyribonuclease B and Antistreptolysin O Titers

Streptococcal antibody tests (eg, antideoxyribonuclease B [ADB] and antistreptolysin O [ASO] titers) are used to confirm previous group A streptococcal infection. The most commonly available streptococcal antibody test is the ASO test. An increase in ASO titers can sometimes be observed but is a late finding and usually of value only in retrospect.

Streptococcal antibody tests can provide confirmatory evidence of recent infection but have no value in acute infection and currently are not indicated in this setting. They may be of value in patients with suspected acute renal failure or acute glomerulonephritis.

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Histologic Findings

The microscopic findings of the eruption of scarlet fever are nonspecific and have an appearance similar to that of other exanthematous eruptions. A sparse neutrophilic perivascular infiltrate is present, with a slight amount of spongiosis in the epidermis. Slight parakeratosis may be present, which probably correlates with the sandpaperlike texture of the skin. The spongiosis and parakeratosis are more noticeable during the desquamative stage. Engorged capillaries and lymphatic dilatation perifollicularly, as well as the presence of dermal hemorrhage and edema, are easily detected.

Gerber MA, Baltimore RS, Eaton CB, Gewitz M, Rowley AH, Shulman ST, et al. Prevention of rheumatic fever and diagnosis and treatment of acute Streptococcal pharyngitis: a scientific statement from the American Heart Association Rheumatic Fever, Endocarditis, and Kawasaki Disease Committee of the Council on Cardiovascular Disease in the Young, the Interdisciplinary Council on Functional Genomics and Translational Biology, and the Interdisciplinary Council on Quality of Care and Outcomes Research: endorsed by the American Academy of Pediatrics. Circulation. 2009 Mar 24. 119(11):1541-51. [Medline].